This invention relates to a laser printer and, more particularly, to an electronic control arrangement for controlling the lengths of the write lines in a laser printer.
It is necessary for a color laser printer to have a print speed substantially the same as a non-color laser printer to have a marketable product. To accomplish this, a color laser printer can utilize four different laser scanner beams to image all four electrophotographic photoconductive drums simultaneously.
When simultaneously imaging the four electrophotographic photoconductive drums, the length of the write line for each of the four laser beams must be substantially equal. This is because the images produced on the four electrophotographic photoconductive drums must overlie each other. If the write lines are not substantially equal, then the four color images will not be in an overlying relation, and the print quality will not be satisfactory.
The primary reason for the four write lines of a color laser printer not being of equal length is the lens magnification in the laser optical systems. This is due to variations in the optical systems of the printer, especially due to thermal effects and variations in the mounting locations of the optical systems relative to the image plane on each of the electrophotographic photoconductive drums.
One previously suggested arrangement for maintaining the write lines substantially equal employed mechanical means for adjusting mirror components in the optical system of each of the four scanning laser beams. This resulted in the lengths of the four write lines being acceptably equal through changing the line magnification. However, this is a relatively expensive and complex solution to the problem.
In a non-color laser printer, it is desired for the write line of a single laser beam to be substantially the same length at all times, particularly when preprinted forms are to be completed by printing on a laser printer. Thermal effects in the laser printer can cause changes in writing line length which can affect locations of print on the preprinted forms.
U.S. Pat. No. 5,117,243 to Swanberg et al discloses the use of PEL slices produced by a clock to control non-linear velocity sweep of a single laser beam along a scan line. U.S. Pat. No. 5,175,636 to Swanberg employs two different clock frequencies to correct for non-linear velocity sweep of a single laser beam along a scan line. However, neither of the aforesaid Swanberg et al and Swanberg patents suggests any correction for lens magnification during operation.
The electronic control arrangement of the present invention satisfactorily solves the foregoing problem of maintaining the lengths of the write lines of a plurality of laser beams of a color laser printer substantially equal. Additionally, this electronic control arrangement also can be utilized in a non-color laser printer to maintain the write lines of one or more laser beams substantially constant.
The electronic control arrangement measures the length of each of the write lines of the laser beams. These measurements are used to adjust the lengths of the write lines so that they are substantially equal.
The differences in the lengths of the write lines of the plurality of laser beams are due to each of the laser beams scanning the electrophotographic photoconductive drum at a different average velocity. That is, the time is the same for each laser beam to write a line of a fixed number of individual print elements (PELs) or dots by scanning one of the electrophotographic photoconductive drums. However, different average velocities of the laser beams in scanning the electrophotographic photoconductive drums to produce the write lines on their surfaces result in the laser beams moving different distances during the same time period to form the write lines.
Each write line scanned by a laser beam is divided into dots or PELs. To discharge an electrophotographic photoconductive drum at desired dot locations, the laser diode is turned on. When no dot is to be printed, the laser diode is turned off. To control the amount of energy delivered to the electrophotographic photoconductive drum to create this latent image, prior art controls the amount of time that the laser beam is turned on and the intensity with which the laser beam is turned on (via the amount of current passed through the laser diode).
To facilitate finer control of the energy delivered to discharge an individual PEL location on the electrophotographic photoconductive drum and of the location of the dot, each of the PELs can be subdivided further into slices, referred to as PEL slices. If, for example, the PEL is divided into eight slices, then turning the laser on for four slices delivers energy over 1/2 (4/8) of the time that the laser beam travels the distance of one PEL (1/600 of an inch or 600 dots per inch) on the electrophotographic photoconductive drum surface. In the preferred embodiment, the PEL is energized at the start of the PEL window, which includes all of the PEL slices comprising the PEL. However, it is not necessary for energization to be at the start of the PEL window.
In addition to ascertaining the average velocity of the laser beam across the electrophotographic photoconductive drum, one embodiment of the electronic control arrangement of the present invention selectively adds PEL slices to the clock timing pulses of a single fixed clock frequency employed for controlling the timing of laser beam energization and for counting the number of PEL slices between the start of scan (SOS) optical sensor and the end of scan (EOS) optical sensor for each of the laser beams. The distance between the SOS sensor and the EOS sensor is a predetermined distance constituting a measuring line. The electronic control arrangement controls when the PEL slices are added so that they are typically added at the end of the PEL where the laser beam is normally off.
In another embodiment, PEL slices may be either added or removed from the clock timing pulses to change the number of the clock timing pulses in a PEL. During factory calibration, a determination is made as to whether the PEL slices will be added or removed during the life of the printer.
The insertion or removal of PEL slices at the same location on each write line may create columns or bands. These columns or bands may be visible on the printed medium depending on the print pattern.
To avoid this possibility, the present invention contemplates offsetting the locations of the insertion or removal of the PEL slices on adjacent write lines. One example would have a constant offset with adjacent write lines having the start of the insertion or removal begin at different PELs. Another example would have different offset values for adjacent write lines. The invention also contemplates changing the locations on a single write line of insertion or removal of PEL slices so that they are not equal.
An object of this invention is to provide substantially equal lengths of laser beam write lines on each of a plurality of photoconductor surfaces.
Another object of this invention is to control the length of a write line on a photoconductor of a laser printer to maintain it substantially the same length at all times.
A further object of this invention is to add or remove PEL slices at selected locations in write lines.
Still another object of this invention is to selectively offset the locations at which PEL slices are added or removed from adjacent write lines.
Other objects of this invention will be readily perceived from the following description, claims, and drawings.